Inhibiting Stress Corrosion Cracking of Metal Exposed to Moderate to High Concentrations of Ethanol

ABSTRACT

An effective stress corrosion cracking (SCC) inhibiting amount of a corrosion inhibitor is added into a blend of fuel and ethanol that contacts a metal, wherein the corrosion inhibitor is an organic acid selected from citric acid, ascorbic acid, succinic acid, pyruvic acid, maleic acid, oxaloacetic acid, oxalosuccinic acid, ketoglutaric acid, isocitric acid, malic acid, aconitic acid, fumaric acid, isomers of these organic acids, and a combination thereof. For example, the corrosion inhibitors inhibit stress corrosion cracking of pipeline grade metal pipe at ethanol concentrations greater than fifteen percent. In one embodiment, the corrosion inhibitor is added into a blend of fuel and ethanol flowing through a pipeline at a plurality of injection points spaced apart along the length of the pipeline. In one option, the corrosion inhibitor is ammoniated to form the ammonium salt of the organic acid. In another option, the foregoing corrosion inhibitors are used in combination with one or more conventional corrosion inhibitors in an amount that is effective to inhibit general corrosion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to co-pending U.S. provisional patent application Ser. No. 61/355,028, filed on Jun. 15, 2010.

BACKGROUND

1. Field of the Invention

The present invention relates to the use of chemical additives to inhibit corrosion of a metal that is exposed to moderate to high concentrations of ethanol, such as in certain blends of transportation fuel and ethanol.

2. Background of the Related Art

Transportation fuels, such as gasoline and diesel, are liquid hydrocarbon mixtures that are used in internal combustion engines. These transportation fuels are produced from crude oil in an oil refinery and distributed to gasoline stations where they are sold to retail consumers in smaller quantities. The oil refineries are typically located where large amounts of crude oil can be easily delivered, such as near a coastline where the crude oil is delivered by large ships. However, gasoline stations that sell both gasoline and diesel are located throughout the regions where fuel is needed for operating automobiles, farm equipment, and other devices having internal combustion engines. Accordingly, gasoline and diesel fuel may be transported from an oil refinery to gasoline stations by truck. Alternatively, gasoline, for example, may be transported from an oil refinery through a pipeline to a regional distribution center before being taken to proximate gasoline stations by truck. These pipeline, as well as storage tanks and other equipment associated with the storage and transportation of gasoline, are at risk for corrosion caused by the fuels or fuel mixtures therein.

Combustion of hydrocarbons in gasoline with oxygen in air produces carbon dioxide, carbon monoxide, water, and various nitrogen oxides. Gasoline may be formulated with other components, such as methyl tertiary butyl ether (MTBE) which raises the octane number and serves as an oxygenate to reduce the amount of carbon monoxide produced. However, MTBE has fallen out of favor as a result of being found as a pollutant in groundwater. Fuel-grade ethanol (FGE) is an alternative oxygenate that is now widely used in gasoline throughout the United States and Brazil. In the U.S., ethanol is primarily produced by the fermentation of yellow corn.

As the use of ethanol and ethanol-gasoline blends increases, it would be desirable to transport these fluids through pipelines. Unfortunately, ethanol can cause corrosion of metal, including stress corrosion cracking (SCC). Furthermore, difficulties arise in transporting ethanol through a pipeline because ethanol has a high affinity for oxygen and water. If any water is present within the pipeline or storage facilities, then the ethanol can become unusable as a transportation fuel. In fact, the extent of the stress corrosion cracking increases with ethanol concentration and the presence of oxygen. For this reason, the ethanol content in much of the gasoline use in the United States does not exceed ten percent (10%).

Though these corrosion issues might be avoided using exotic metallurgies or polymer lined pipe, the investment necessary to build new pipelines with the necessary materials is prohibitively expensive. Rather, it would be desirable to use the existing infrastructure of pipelines, or even new pipelines made with standard pipeline-grade steel, to handle ethanol or ethanol/gasoline blends. Still further, it would be beneficial to enable pipelines to handle ethanol at concentrations greater than ten percent. Presently used transport fuel corrosion inhibitors, often referred to as “rust” inhibitors, as qualified using the procedure written in NACE Standard TM0172, have not been demonstrated to be effective at stopping stress corrosion cracking caused by ethanol in pure ethanol or in ethanol blends. The chemistries used for these rust inhibitors are known as: dimer acids, trimer acids, or blends of dimer and trimer acids; and derivatives of succinic anhydride.

BRIEF SUMMARY

One embodiment of the present invention provides a method for inhibiting corrosion of metal exposed to a blend of fuel and ethanol. The method comprises adding an effective stress corrosion cracking inhibiting amount of a corrosion inhibitor into the blend of fuel and ethanol that contacts the metal, wherein the corrosion inhibitor is an organic acid selected from citric acid, ascorbic acid, succinic acid, pyruvic acid, maleic acid, oxaloacetic acid, oxalosuccinic acid, ketoglutaric acid, isocitric acid, malic acid, aconitic acid, fumaric acid, isomers of these organic acids, and a combination thereof.

Another embodiment of the present invention provides a further method for inhibiting corrosion of metal exposed to a blend of fuel and ethanol. The method comprises adding an effective stress corrosion cracking inhibiting amount of a corrosion inhibitor into the blend of fuel and ethanol that contacts the metal, wherein the corrosion inhibitor is an organic acid having one or more functional groups selected from carboxylic acids, alkene bonds, hydroxyl groups, and combinations thereof.

DETAILED DESCRIPTION

The present invention provides a method for inhibiting corrosion of metal in a pipeline containing a blend of gasoline and ethanol. The method comprises adding an effective corrosion inhibiting amount of a corrosion inhibitor into a blend of gasoline and ethanol flowing through a pipeline, wherein the corrosion inhibitor is an organic acid selected from citric acid, ascorbic acid, succinic acid, pyruvic acid, maleic acid, oxaloacetic acid, oxalosuccinic acid, ketoglutaric acid, isocitric acid, malic acid, aconitic acid, fumaric acid, isomers of these organic acids, and a combination thereof.

The present invention may be used with any ethanol concentration, but the corrosion inhibitors are effective where the ethanol concentration in the blend of fuel and ethanol is greater than ten percent, greater than fifteen percent, greater than twenty-five percent, or even greater than 95 percent. For example, the corrosion inhibitors are effective for inhibiting stress corrosion cracking of metal exposed to high concentrations of ethanol, such as in the fuel mixture known as E85 containing up to 85% ethanol and gasoline.

In one embodiment, the selected organic acid is ammoniated. For example, the organic acid may be mixed with ammonium hydroxide in substantially stoichiometric amounts such that the organic acid is present as the ammonium salt of the organic acid.

In another embodiment, the corrosion inhibitor is added into the blend of fuel and ethanol in an amount providing between 10 and 1000 ppm of the corrosion inhibitor based on the ethanol content of the blend. More specifically, the corrosion inhibitor may be added into the blend in an amount providing between 100 and 600 ppm of the corrosion inhibitor based on the ethanol content of the blend. However, it is believed that the corrosion inhibitors are equally effective regardless of whether they are added before or after the blending of the ethanol and fuel.

In a further embodiment, the corrosion inhibitor is optionally added as a solution including a solvent selected from water, methanol, and combinations thereof. The solvent delivery system used with the corrosion inhibitor is not believed to play any role in the performance of the corrosion inhibitor in inhibiting stress corrosion cracking, but should not interfere with the ultimate use of the fuel blend.

In a still further embodiment, the method includes flowing the blend of fuel and ethanol through a pipeline including metal exposed to the blend of fuel and ethanol. Accordingly, the corrosion inhibitor is added into the flowing blend of fuel and ethanol at a plurality of injection points spaced apart along the length of a pipeline. Adding the corrosion inhibitor in this manner preferably enables a sufficient concentration of the corrosion inhibitor throughout the length of the pipeline.

In a similar embodiment, the method may include storing the blend of fuel and ethanol in a storage tank including metal exposed to the blend of fuel and ethanol. In yet another embodiment, the foregoing corrosion inhibitors are used in combination with one or more other corrosion inhibitors. It is believed that the corrosion inhibitors of the present invention are effective for inhibiting ethanol-induced stress corrosion cracking, whereas conventional corrosion inhibitors may also be used to inhibit corrosion caused by other components flowing in the pipe and/or corrosion of other types or mechanisms. For example, 500 ppm citric acid may be added into the blend of ethanol and fuel to inhibit stress corrosion cracking, while one or more conventional corrosion inhibitors may also be added into the blend of ethanol and gasoline to inhibit general corrosion or pitting. For example, a conventional corrosion inhibitor may include a sulfur-containing functional group (such as a mercapto or thiol) or a quaternary amine functional group. Alternatively, the conventional corrosion inhibitor may be an imidazoline corrosion inhibitor. Still further, the conventional corrosion inhibitor may be selected from dimer acids, trimer acids, derivatives of succinic anhydride, and combinations thereof. The conventional corrosion inhibitors may be used in one of the foregoing methods further comprising the step of adding an effective general corrosion inhibiting amount of a conventional corrosion inhibitor into the blend of fuel and ethanol.

Example Evaluation of Inhibitors for Preventing SCC in Existing and New Pipelines

Both slow strain rate (SSR) and crack growth tests were performed on base metal specimens machined from one X-60 line pipe steel to illustrate the effectiveness of various inhibitors at preventing ethanol stress corrosion cracking (SCC). The testing was performed with un-notched specimens having a gage length of 25 mm (1 inch) and a gage diameter of 3.2 mm (0.125 inches). A displacement rate of 1×10⁻⁶ inches/sec was used, which produced a strain rate of 1×10⁻⁶ sec⁻¹. The SSR tests were performed in stainless steel test cells with a total volume of 400 mL, where the volume was filled with 350 mL of solution leaving a vapor space of 50 mL.

The tests were performed using a simulated fuel grade ethanol (SFGE) containing 5 ppm chloride (Cl). The specimens were tested under freely corroding conditions and the corrosion potential was monitored in each test using an Ag/AgCl/EtOH reference electrode. Based on independent measurements of chloride leakage rate from the reference electrode, it was estimated that the chloride concentration in the test cell increased by about 1 ppm during the course of the SSR tests. A piece of rusted pipe steel was placed in the test cell and galvanically connected to the test specimen to more closely simulate the native corrosion potential of a mill scaled/rusted pipe wall. The rusted steel to specimen area ratio was approximately 5 to 1. The specimen and rusted steel piece were electrically isolated from the specimen grips and test cell in the SSR test machine. The tests were performed at room temperature and the cell was actively sparged with breathing air at a flow rate of approximately 4 mL/minute. Ethanol bubbler traps were used on the inlet and outlet to the test cell to remove/exclude any moisture. Post-test analysis was not performed on the test solutions, but extensive previous water analyses of test solutions from the SSR tests indicated that there was negligible pick-up of water in the tests.

After testing, the specimens were examined and optically photographed. The fracture surfaces were examined in a scanning electron microscope (SEM) and the depths of the stress corrosion cracks on the fracture surfaces were measured. The depth of the second deepest crack in each specimen was recorded. Other parameters that were recorded for each test included the time to failure, ultimate tensile strength (UTS), and reduction in area (un-notched specimens only).

Table 1 summarizes the results of the SSR tests. The first column in the table identifies the conditions to which the specimen was exposed, including air, FGE, SFGE and various inhibitors. The second column in the table is the total time to failure in the SSR test, in hours. The third column in the table is the reduction in area of the cross section of the specimen, in percent. In general, less reduction in area occurs in the smooth tensile specimens that exhibit cracking. The fourth column in the table is the maximum stress sustained by the specimen, the ultimate tensile strength (UTS), which is the maximum load divided by the initial cross sectional area.

The fifth column in Table 1 is the time-to-failure ratio, which is the time to failure for each test divided by the average time to failure for the duplicate air tests of specimens of the same material. The sixth column in the table is the reduction in area ratio (for un-notched specimens), which is the reduction in area for each test specimen divided by the average reduction in area for the air tests. The seventh column in the table is the UTS ratio, which is the UTS for each test specimen divided by the average UTS for the air tests.

The eighth column in Table 1 is the crack depth, in micrometers, measured on the fracture surface in the SEM. The depth of the second deepest thumbnail crack on the fracture surface of each specimen is recorded, since in some cases the deepest crack was not representative of the other cracks found on the fracture surface. The ninth column in the table is the pseudo-crack growth rate, in mm/s, calculated by dividing the crack depth by the time to failure. No change (“-”) in columns eight and nine indicates that no cracking was found in the specimen.

The time to failure, reduction in area, UTS, ratios of these parameters to the air tests, crack depth, and crack growth rate all provide indications of the severity of SCC that occurred in the specimen. In general, the time to failure ratio, reduction in area ratio, and UTS ratios are preferable to the underlying parameters since they more easily indicate cracking severity. The smaller the value of the ratio relative to one, the more severe the cracking.

The tenth column in Table 1 is the average corrosion potential. In each test, corrosion potential readings were recorded every minute using a data acquisition system. The potential data for each test were averaged and the results are shown in Column 10.

The data from the control tests of line pipe steel in air (the first and second rows of the table) were used to calculate the mechanical property ratios for the SSR tests performed in the cracking environments.

The results of the baseline test performed in the SFGE (fifth row) show that the time to failure ratio and reduction in area ratio were all less than the values in the air tests and relatively deep stress corrosion cracks were present on the fracture surface. The results of the baseline test performed in one lot of actual FGE (fourth row) was very similar to the results of the baseline test performed in the SFGE (fifth row). This indicated that the extent of SCC in this specimen exposed to FGE was very similar to that observed in the specimen exposed SFGE, thereby validating the use of SFGE for most of the inhibitor work.

The same tests were repeated on identical specimens using a citric acid inhibitor formulation, glucose in methanol/water inhibitor formulation, ammoniated L-ascorbic acid in water inhibitor formulation, and L-ascorbic acid in water inhibitor formulation. No SCC was observed with two of the inhibitors (citric acid and ammoniated L-ascorbic acid), but there was some pitting on the gage section of the specimen tested in citric acid. Minor SCC was observed with L-ascorbic acid, while the glucose inhibitor was the only inhibitor in which significant SCC was observed. Based on these results, citric acid performed very well and the pitting associated with citric acid may be eliminated with the use of a conventional corrosion inhibitor. L-ascorbic acid alone was approximately 80% effective and with the addition of ammonia it was nearly 100% effective to inhibit SCC in line pipe steel exposed to FGE.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components and/or groups, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 

1. A method for inhibiting corrosion of metal exposed to a blend of fuel and ethanol, comprising: adding an effective stress corrosion cracking inhibiting amount of a corrosion inhibitor into the blend of fuel and ethanol that contacts the metal, wherein the corrosion inhibitor is an organic acid selected from citric acid, ascorbic acid, succinic acid, pyruvic acid, maleic acid, oxaloacetic acid, oxalosuccinic acid, ketoglutaric acid, isocitric acid, malic acid, aconitic acid, fumaric acid, isomers of these organic acids, and a combination thereof.
 2. The method of claim 1, wherein the fuel is selected from gasoline and diesel.
 3. The method of claim 1, wherein the corrosion inhibitor is added into the blend of fuel and ethanol in an amount providing between 10 and 1000 ppm of the corrosion inhibitor based on the ethanol content of the blend.
 4. The method of claim 1, wherein the corrosion inhibitor is added into the blend of fuel and ethanol in an amount providing between 100 and 600 ppm of the corrosion inhibitor based on the ethanol content of the blend.
 5. The method of claim 1, wherein the corrosion inhibitor is added as a solution including a solvent selected from water, methanol, and combinations thereof.
 6. The method of claim 1, wherein the ethanol concentration in the blend of fuel and ethanol is greater than fifteen percent.
 7. The method of claim 1, wherein the ethanol concentration in the blend of fuel and ethanol is greater than twenty-five percent.
 8. The method of claim 1, further comprising: flowing the blend of fuel and ethanol through a pipeline including metal exposed to the blend of fuel and ethanol.
 9. The method of claim 8, wherein the corrosion inhibitor is added into the flowing blend of fuel and ethanol at a plurality of injection points spaced apart along the length of the pipeline.
 10. The method of claim 1, further comprising: storing the blend of fuel and ethanol in a storage tank including metal exposed to the blend of fuel and ethanol.
 11. The method of claim 1, wherein the corrosion inhibitor includes citric acid.
 12. The method of claim 1, wherein the corrosion inhibitor includes ascorbic acid.
 13. The method of claim 1, wherein the corrosion inhibitor is ammoniated to form the ammonium salt of the organic acid.
 14. The method of claim 1, further comprising: adding an effective general corrosion inhibiting amount of a conventional corrosion inhibitor into the blend of fuel and ethanol.
 15. The method of claim 14, wherein the conventional corrosion inhibitor is selected from dimer acids, trimer acids, derivatives of succinic anhydride, and combinations thereof.
 16. A method for inhibiting corrosion of metal exposed to a blend of fuel and ethanol, comprising: adding an effective stress corrosion cracking inhibiting amount of a corrosion inhibitor into the blend of fuel and ethanol that contacts the metal, wherein the corrosion inhibitor is an organic acid having one or more functional groups selected from carboxylic acids, alkene bonds, hydroxyl groups, and combinations thereof.
 17. The method of claim 16, wherein the ethanol concentration in the blend of fuel and ethanol is greater than fifteen percent.
 18. The method of claim 16, wherein the corrosion inhibitor is ammoniated to form the ammonium salt of the organic acid.
 19. The method of claim 16, further comprising: flowing the blend of fuel and ethanol through a pipeline including metal exposed to the blend of fuel and ethanol.
 20. The method of claim 19, wherein the corrosion inhibitor is added into the flowing blend of gasoline and ethanol at a plurality of injection points spaced apart along the length of the pipeline.
 21. The method of claim 16, further comprising: storing the blend of fuel and ethanol in a storage tank including metal exposed to the blend of fuel and ethanol.
 22. The method of claim 16, further comprising: adding an effective general corrosion inhibiting amount of a convention corrosion inhibitor into the blend of fuel and ethanol.
 23. The method of claim 22, wherein the conventional corrosion inhibitor is selected from dimer acids, trimer acids, derivatives of succinic anhydride, and combinations thereof. 